For keeping the environment of the earth, use of solar energy is getting popular. Albeit solar cell modules including solar cell elements in which a plurality of solar cells are linked are implemented on more and more roofs or walls of ordinary buildings and houses, high cost of the solar cell made of Si (silicon) or the like hinders wide use of the solar cell.
One cause of the high cost of the solar cell is a need of a step of evaluating an output characteristics thereof regarding an output thereof made when sun light is irradiated on the solar cell module, that is a need of step of evaluating characteristics of the solar cell module. The step of evaluating the output characteristics of the solar cell module measures an important item in post-production inspection of the solar cell module and in research and development of solar cells.
In general, it is difficult to evaluate the output characteristics of the solar cell module by irradiating sun light directly on the solar cells, for example, because intensity of the sun light varies depending on weather. On this account, the step of evaluating the output characteristics of the solar cell module generally uses a so-called solar simulator, that is, a light source typically using Xe (xenon) lamp or halogen lamp as a light source for evaluating the output characteristics of the solar cells, instead of actual sun light (see Patent Citation 1 for example). Moreover, a method for evaluating the solar cells by using an LED as a light source instead of the Xe lamp or halogen lamp (see Patent Citation 2 for example) has been developed.
Meanwhile, so-called EBIC (Electron Beam Induced Current) and LBIC (Laser Beam Induced Current), that is, methods for measuring a current or voltage induced by using an electron beam or laser beam and thereby analyzing diffusion length of minority carriers and defects (grain boundary/transgranular), are widely used as alternative methods for evaluating the performance of the solar cells by using the solar simulator.
By the EBIC or LBIC, it is possible to measure and evaluate a degree of an elective activity or diffusion length of the minority carriers in solar cells locally. It is possible to evaluate conversion efficiency and quality of the solar cell based on the result thereof (Non-Patent Citation 1).
Furthermore, because a solar cell has a pn junction and a structure similar to that of a light emitting diode (LED). A solar cell element (InGaP/GaAs) including a gallium arsenide monocrystalline semiconductor, which is generally using in the LED, has been developed. Techniques for evaluating such a solar cell element including gallium arsenide monocrystalline semiconductor have been reported. The evaluation is carried out by applying on the solar cell element a bias in a forwarding direction so as to cause electroluminescence (EL) and observe the EL. In-plane Unevenness in an EL intensity due to uneven current density distribution and pn junction-leakage-causing defect are evaluated based on the observation. (For example, see Non-Patent Citations 2 and 3.)
[Patent Citation 1]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-48704 (published on Feb. 15, 2002).
[Patent Citation 2]
Japanese Unexamined Patent Application Publication, Tokukai, No. 2004-281706 (published on Oct. 7, 2004).
[Non-Patent Citation 1]
N. Sakitani, et al., “Evaluation of Recombination Velocity at Grain Boundaries in Poly-Si Solar Cells with Laser Beam Induced Current” Solid State Phenomena Vol. 93 (2003), pp. 351-354
[Non-Patent Citation 2]
Tatsuya Takamoto, et al., “STUDY ON PERFORMANCE UNIFORMITY OF InGaP/GaAs TANDEM SOLAR CELLS BY USING PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE TECHNIQUES”, presented at the 14th European Photovoltaic Solar Energy Conference, 30 Jun.-4 Jul., 1997 in Barcelona, Spain
[Non-Patent Citation 3]
Tatusya, TAKAMOTO, Thesis for a doctorate (engineering) “Research on Improvement of InGaP/GaAs tandem structure solar cell in efficiency and its property”, Toyota Technological Institute, Postgraduate School of Engineering, submitted January, 1999.
The solar simulator and apparatuses for EBIC or LBIC are however, large in size and requires high equipment investment. Moreover, there is such a problem that the evaluation methods using these apparatus are actually complicated. That is, evaluation cost in the production cost of the solar cells has not been lowered due to high equipment cost of the measuring apparatuses for measuring and evaluating the performance of the solar cell element, and time-consuming measurement process.
Moreover, shipping inspection of solar cells by the solar simulator should measure an output current or output voltage of the whole solar cell module, thereby inspecting photoelectric conversion efficiency of the whole solar cell module. This cannot perform such a detailed analysis to find out which solar cell element at which portion of the solar cell module has a poor conversion efficiency.
Furthermore, the apparatus for EBIC or LBIC has many facility restrictions such as a need of an electron microscope for radiating the electron beam, or a multi-wavelength light source for radiating the laser beam. Therefore, it is not easy to perform this evaluation method for evaluating the solar cell module.
While the solar cell element made of a silicon-based material is most popular at present, a technique for evaluating the performance of such a solar cell element has not been developed albeit the technique for evaluating, by an EL method, the solar cell element made of monocrystals of InGaP/GaAs in terms of in-plane unevenness in EL intensity and in terms of the defect that will cause leakage in pn junction.
InGaP/GaAs has been generally used as a raw material for LED, and it has been a well known fact that it is easy to cause InGaP/GaAs to emit light by biasing InGaP/GaAs in the forwarding direction. Therefore, it has been relatively easy to arrive such a concept to evaluate the performance of the solar cell element by EL method. Silicon semiconductor, by contrast, has not been reported that it emits light under normal conditions while silicon semiconductor, even though it is an indirect transition semiconductor, emits light due to light emitting transition (caused by hot carrier etc.) or braking radiation under special conditions (such as being under low temperatures or under high electric field application, etc.) and there are reported a few cases that such light emission is utilized to perform property evaluation or performance evaluation. Therefore, silicon semiconductor has not been used as a light emitting material for LED or the like.
As described above, the light emitting characteristics of silicon semiconductor is not sufficient and silicon semiconductor is clearly different from GaAs in terms of its property. On this account, the knowledge regarding InGaP/GaAs cannot be directly applied to silicon semiconductor.
Furthermore, the knowledge regarding InGaP/GaAs disclosed in Non-Patent Citations 2 and 3 discuss about the solar cell element made of monocrystals of gallium arsenide semiconductor. Monocrystalline semiconductors have such an electron property that in-plane distribution does not occur and that makes it possible to form elements thereof with uniform property. Because of this, an element made of a monocrystalline semiconductor can be evaluated with no particular difficulty. However, polycrystalline semiconductors have such an electron property that in-plane distribution occurs, and that makes it possible to form elements only with largely varied properties. Thus, the use of a polycrystalline semiconductor requires more accurate evaluation.
Therefore, there has been a demand for development of a method and apparatus for evaluating a solar cell, each of which make it possible to evaluate photoelectric conversion performance of a solar cell module easily and accurately, and use thereof. Especially, silicon polycrystalline solar cell has been rapidly advanced to practical use. There is an immediate demand for development of an evaluation method etc. to contribute to high performance thereof.